Syria says it will punish its former envoy to Iraq who fled to Qatar and denounced the regime of Bashar al-Assad, then called on other ambassadors to do the same.

The defection of Nawaf al-Fares, who was until Wednesday a leading member of Syria's diplomatic corp, has drawn an angry response from Damascus, which claims it had in fact sacked Fares and says he will face "judicial" measures.

Fares is a leading tribal member from Syria's eastern desert region and is the most significant figure to flee the regime in almost 17 months of uprising.

"Every Syrian man has to join the revolution to remove this nightmare and this gang," he said. The main opposition group, the Syrian National Council claims it has been talking to other Syrian diplomats who will soon follow suit.

The French foreign minister, Kaurent Fabius, said a dialogue had started between a second high-profile defector, Brigadier General Manaf Tlass, who had been regarded as a confidant and friend of the Assad family until he fled to Turkey last Thursday.

Meanwhile, the international diplomatic response to the escalating crisis in Syria remains in gridlock with Russia and China continuing to oppose a western move to press the United Nations to threaten Damascus with sanctions, before 20 July.

On that date the mandate expires for the current UN mission which, despite a 90-day remit, has failed to stem a mounting casualty toll in Syria, where daily death tolls now often top 100.

Syria's allies have agreed to an extension of the mission, but have refused to support a UN resolution that allows for the threat of new sanctions.

Western nations want to ensure that a 10-day ultimatum to Syria to withdraw its heavy weapons and pull back from towns and cities is a central part of a new resolution which must be passed before the current mission's term expires next Friday.

The backer of the original UN mission and the peace plan that accompanied it, Kofi Annan, has warned of "clear consequences" if a ceasefire does not take root this time

Annan has returned from visits to Damascus, as well as Tehran and Baghdad, where he has tried to secure support for a new attempt to establish a ceasefire in Syria.

He characterised his meeting with Assad as "constructive" and leaked minutes from the talks appear to show that both men will aim for a ceasefire to cascade from an initial detente in selected areas.

Violence in Syria has shown no signs of abating despite the talks, with clashes continuing in the north and west of the country and mortars reportedly used on the outskirts of Damascus – the first attack so close to the regime's power base.